Field of the Invention
The invention relates to a metallic pipe comprising a hydrolysis-resistant layer of a polyamide moulding composition, to the use of this type of pipe for conveying a hydrolyzing medium, to a hydrolysis-resistant polyamide moulding composition suitable for contact with a hydrolyzing medium, and to mouldings made therefrom.
Discussion of the Background
The property profile of polyamides, and in particular of polyamides with a low concentration of carbonamide groups, for example PA11 and PA12, has led to use of these in a wide variety of technical application sectors. Among these, mention may be made inter alia of lines for the transport of coolants in the automobile industry, and also inliners in the field of offshore oil production, which in particular require good hydrolysis resistance. However, applications of this type increasingly demand materials with even higher hydrolysis resistance, in particular at higher temperatures.
U.S. Pat. No. 5,850,855 discloses, for automobiles, a coolant-liquid line in which the external layer is composed of a polyamide with an excess of terminal amino groups. The intention there is to improve hydrolysis resistance. However, pipes of this type produced by coextrusion use a moulding composition with a high melt viscosity, and this means that because of the high molecular weight associated therewith the polyamide has a low concentration of terminal groups. When an excess of terminal amino groups is present here, it is insufficiently effective simply in terms of quantity. This solution is moreover restricted to pipes with small diameter, of the type typical in automobile applications.
When pipes, profiles and other hollow bodies are extruded, in particular in geometries having large dimensions, various problems can arise after discharge of the melt from the die, these being caused inter alia by gravitational force. A visual sign of low melt viscosity here is that the melt tube sags as it is discharged. Gravity causes wall thickness changes, and the hollow body can therefore have irregular wall thickness distribution. There are moreover severe restrictions on the sizes achievable for particular geometries and on the geometries achievable in profile extrusion. The melt stiffness of conventional polyamides is not adequate here to permit production of the preferred geometries in a manner that is technically and economically satisfactory and that provides dimensional accuracy and reliability. Low melt stiffness moreover leads to an uneven, unstable extrusion process, a result of which can be uneven progress of the melt strand into the calibration unit. This can lead to production problems. If, in contrast, the melt tube has high melt stiffness after it leaves the die, its progress is markedly more stable and it becomes less susceptible to external extrusion effects. In the case of vertical extrusion (e.g. parison) it is necessary to prevent break-off of the extruded melt tube, and also to prevent any increase in length, since this would reduce wall thickness. The dimensions of the geometries that this extrusion technology can produce are currently limited by the melt stiffness of the polyamide moulding composition used. Specifically, there is a requirement here for high melt stiffness in order to permit extrusion of large dimensions.
However, high viscosity makes it difficult to extrude a polyamide moulding composition with high melt stiffness. This requires build-up of an exceptionally high pressure in the machine; even if this is achieved it remains impossible to produce geometries with large dimensions at extrusion rates that are economically viable, since even at relatively small throughputs the motor is subjected to high load.
U.S. Pat. No. 8,647,551 and U.S. Pat. No. 8,580,899 provide a solution to this problem. Those specifications describe a process for the production of mouldings with condensation of a polyamide moulding composition to increase molecular weight, with a compound having at least two carbonate units, where a premix of the polyamide moulding composition and the compound having at least two carbonate units is produced and the premix is then processed to give the moulding, and it is in this step that the melting of the premix and the condensation to increase molecular weight first take place. US 2011/0165358 moreover discloses that a compound having at least two carbonate units can be used in a masterbatch which moreover comprises a polyetheramide, at least 50% of the terminal groups of which are amino groups. This process has the disadvantage that terminal amino groups are consumed in the reaction of the compound having at least two carbonate units with the polyamide. These are then no longer available for hydrolysis resistance.
U.S. Pat. No. 6,677,015 describes a moulding composition with good capability for blow-moulding; it is composed of at least 50% by weight of a polyamide produced with use of from 0.05 to 2.5% by weight of a polyamine as branching agent. This moulding composition exhibits a high level of pseudoplasticity; however that specification describes only the production of relatively small-volume mouldings.
Moulding compositions similar to those of U.S. Pat. No. 6,677,015 can, according to US 2010/0009106, US 2010/0300573, US 2011/0209768 and US 2011/0165358, be processed together with a compound having at least two carbonate units to give large pipes which are used as inliners for pipelines, for example supply lines, drainage lines or oil-conveying lines. The polyamide moulding compositions used there can comprise a polyamine as branching agent; however, no moulding compositions of that type are available commercially.
Hydrolysis-resistant mouldings made of a polyamide moulding composition are known from US 2013/0171388. That document says that a premix of a polyamide moulding composition and from 0.1 to 5% by weight of an oligo- or polycarbodiimide is processed to give a moulding, and condensation to increase molecular weight, i.e. an increase in the molecular weight of the polyamide, takes place in the processing step here. It is thus possible to produce mouldings having markedly improved hydrolysis resistance, with geometries having large dimensions.
For many applications, however, the hydrolysis resistance achievable according to the background art in polyamide moulding compositions is not sufficient. In particular in the case of oil-conveying lines, the lifetime of the inliners present therein is a decisive factor for cost-effectiveness. This is particularly the case for the flexible offshore lines that are very expensive because of their complicated structure. When the inliner fails it cannot readily be replaced. The requirement that has to be met is therefore that the operating time of the inliner at least equals the intended service time of the line.